A new purple sulfur bacterium from stratified freshwater lakes,Amoebobacter purpureus sp. nov.

Six strains of a new purple sulfur bacterium were isolated from the chemocline of four different freshwater lakes. Single cells were spherical to oval, nonmotile and contained gas vacuoles in the central part of the cytoplasm. All strains contained bacteriochlorophyll a and okenone as the major carotenoid. The intracytoplasmic membrane system was of vesicular type. All strains resembled each other in growth conditions and utilization of simple organic carbon sources. The strains were able to grow microaerophilic in the dark, used hydrogen sulfide, elemental sulfur or thiosulfate as electron donor, and lacked assimilatory sulfate reduction. On the basis of all characteristics the new bacterium represents a new species of the genus Amoebobacter, A. purpureus sp. nov.     

Desulfuromonas acetoxidans gen. nov. and sp. nov., a new anaerobic,sulfur-reducing,acetate-oxidizing bacterium

Anaerobic sea or fresh water media with acetate and elemental sulfur yielded enrichments of a new type of strictly anaerobic, rod-shaped, laterally flagellated, Gram-negative bacterium. Three pure culture-strains from different sulfide-containing sea water sources were characterized in detail and are described as a new genus and species Desulfuromonas acetoxidans.The new bacterium is unable to ferment organic substances; it obtains energy for growth by anaerobic sulfur respiration. Acetate, ethanol or propanol can serve as carbon and energy source for growth; their oxidation to CO₂ is stoichiometrically linked to the reduction of elemental sulfur to sulfide. Organic disulfide compounds, malate or fumarate are the only other electron acceptors used. Butanol and pyruvate are used in the presence of malate only; no other organic compounds are utilized. Biotin is required as a growth factor. The following dry weight yields per mole of substrate are obtained: in the presence of sulfur: 4.21 g on acetate, 9.77 g on ethanol; in the presence of malate: 16.5 g on acetate, 34.2 g on ethanol and 46.2 g on pyruvate. Accumulations of cells are pink; cell suspensions exhibit absorption spectra resembling those of c-type cytochromes (abs. max. at 419, 523 and 553 nm). Malate-ethanol grown cells contain a b-type cytochrome in addition.In the presence of acetate, ethanol or propanol, Desulfuromonas strains form robust growing syntrophic mixed cultures with phototrophic green sulfur bacteria.Dedicated to Prof. Roger Y. Stanier on the occasion of his 60th barthday     

Fermentative degradation of dipicolinic acid (pyridine-2,6-dicarboxylic acid) by a defined coculture of strictly anaerobic bacteria

Degradation of dipicolinic acid (pyridine-2,6-dicarboxylic acid) under strictly anaerobic conditions was studied in enrichment cultures from marine and freshwater sediments. In all cases, dipicolinic acid was completely degraded. From an enrichment culture from a marine sediment, a defined coculture of two bacteria was isolated. The dipicolinic acid-fermenting bacterium was a Gram-negative, non-sporeforming strictly anaerobic short rod which utilized dipicolinic acid as sole source of carbon, energy, and nitrogen, and fermented it to acetate, propionate, ammonia, and 2CO₂. No other substrate was fermented. This bacterium could be cultivated only in coculture with another Gram-negative, non-sporeforming rod from the same enrichment culture which oxidized acetate to CO₂ with fumarate, malate, or elemental sulfur as electron acceptor, similar to Desulfuromonas acetoxidans. Since this metabolic activity is not important in substrate degradation by the coculture, the basis of the dependence of the dipicolinic acid-degrading bacterium on the sulfur reducer may be sought in the assimilatory metabolism.     

Growth of sulfate-reducing bacteria with sulfur as electron acceptor

In addition to three new isolates, six strains of representative species of sulfate-reducing bacteria were tested for their capacity to use elemental sulfur as an electron acceptor for growth. There was good growth and sulfide production by strain Norway 4 and the three isolates, two of which had been enriched with sulfur flower and one isolated from a culture with green sulfur bacteria. Slow but definite growth was observed with Desuflovibrio gigas. The type strains of Desulfovibrio desulfuricans, D. vulgaris, and Desulfotomaculum nigrificans as well as Desulfomonas pigra did not grow with sulfur. The four strains that grew well with sulfur flower were straight, nonsporulating rods and did not contain desulfoviridin.     

Mechanism of acetate oxidation to CO2 with elemental sulfur in Desulfuromonas acetoxidans

The strict anaerobe Desulfuromonas acetoxidans can oxidize acetate to CO₂ with elemental sulfur as electron acceptor. ¹⁴C-labelling experiments and enzyme studies are described revealing that acetate oxidation proceeds via the citric acid cycle with the synthesis of oxaloacetate from acetate and 2 CO₂ via pyruvate as anaplerotic reaction. An oxidation of acetate via one carbon unit intermediates as proposed for anaerobic bacteria fermenting acetate to 2 CO₂ and 4 H₂ was excluded.Dedicated to Professor Dr. Gerhart Drews on the occasion of his 60th birthday     

Anaerobic degradation of 1,3-propanediol by sulfate-reducing and by fermenting bacteria

Three strains of strictly anaerobic Gram-negative, non-sporeforming, motile bacteria were enriched and isolated from freshwater sediments with 1,3-propanediol as sole energy and carbon source. Strain OttPdl was a sulfate-reducing bacterium which grew also with lactate, ethanol, propanol, butanol, 1,4-butanediol, formate or hydrogen plus CO₂, the latter only in the presence of acetate. In the absence of sulfate, most of these substrates were fermented to the respective fatty acids in syntrophic cooperation with Methanospirillum hungatei. Sulfur, thiosulfate, or sulfite were reduced, nitrate not. The other two isolates degraded propanediol only in coculture with Methanospirillum hungatei. Strain OttGlycl grew in pure culture with acetoin and with glycerol in the presence of acetate. Strain WoAcl grew in pure culture only with acetoin. Both strains did not grow with other substrates, and did not reduce nitrate, sulfate, sulfur, thiosulfate or sulfite. The isolates were affiliated with the genera Desulfovibrio and Pelobacter. The pathways of propanediol degradation and the ecological importance of this process are discussed.     

Nitrogen fixation and nitrate utilization by marine and freshwater Beggiatoa

Four newly isolated marine strains of Beggiatoa and five freshwater strains were tested for nitrogen fixation in slush agar medium. All strains reduced acetylene when grown microaerobically in media containing a reduced sulfur source and lacking added combined nitrogen. The addition of 2 mmol N, as nitrate or ammonium salts, completely inhibited this reduction. Although not optimized for temperature or cell density, acetylene reduction rates ranged from 3.2 to 12 nmol·mg prot^-1 min^-1. Two freshwater strains did not grow well or reduce acetylene in medium lacking combined nitrogen if sulfide was replaced by thiosulfate. Two other strains grew well in liquid media lacking both combined nitrogen and reduced sulfur compounds but only under lowered concentrations of air. All freshwater strains grew well in medium containing nitrate as the combined nitrogen source. Since they did not reduce acetylene under these conditions, we infer that they can assimilate nitrate.     

Studies on dissimilatory sulfate-reducing bacteria that decompose fatty acids II. Incomplete oxidation of propionate by Desulfobulbus propionicus gen. nov., sp. nov.

A new type of sulfate-reducing bacteria with ellipsoidal to lemon-shaped cells was regularly enriched from anaerobic freshwater and marine mud samples when mineral media with propionate and sulfate were used. Three strains (1pr3, 2pr4, 3pr10) were isolated in pure culture. Propionate, lactate and alcohols were used as electron donors and carbon sources. Growth on H₂ required acetate as a carbon source in the presence of CO₂. Stoichiometric measurements revealed that oxidation of propionate was incomplete and led to acetate as an endproduct. Instead of sulfate, strain 1pr3 was shown to reduce sulfite and thiosulfate to H₂S; nitrate also served as electron acceptor and was reduced to ammonia. With lactate or pyruvate, all three strains were able to grow without external electron acceptor and formed propionate and acetate as fermentation products. None of the strains contained desulfoviridin. In strain 1pr3 cytochromes of the b- and c-type were identified. Strain 1pr3 is described as type strain of the new species and genus, Desulfobulbus propionicus.     

Different mechanisms of acetate activation in Desulfurella acetivorans and Desulfuromonas acetoxidans

Desulfurella acetivorans and Desulfuromonas acetoxidans are both acetate oxidizing sulfur reducing eubacteria. The two organisms differ in G+C content of DNA (31.4% versus 50–52%) and in growth temperature optimum (55°C versus 30°C) and in that D. acetivorans does not contain cytochromes. Both organisms are shown to be similar in that they metabolize acetate via the citric acid cycle rather than via the carbon monoxide dehydrogenase pathway. They were found to differ, however, in the mechanism of acetate activation and of succinate formation. In D. acetoxidans acetyl-CoA and succinate are formed from acetate and succinyl-CoA involving only one enzyme, succinyl-CoA: acetate CoA-transferase. In D. acetivorans acetyl-CoA is generated from acetate via acetyl phosphate involving acetate kinase and phosphate acetyltransferase; succinate is formed from succinyl-CoA via succinyl-CoA synthetase. Both sulfur reducers were found to contain menaquinone.Abbreviations HPLC high performance liquid chromatography - acetyl-P acetyl phosphate     

Photoproduction of H2 from acetate by syntrophic cocultures of green sulfur bacteria and sulfur-reducing bacteria

The marine green sulfur bacterium Chlorobium vibrioforme strain 1930 produced H₂ and elemental sulfur from sulfide or thiosulfate under N limitation in the light. H₂ production depended on nitrogenase and occurred only in the absence of ammonia. Methionine sulfoximine, an inhibitor of glutamine synthetase, prevented the switch-off by ammonia. In defined syntrophic cocultures of the acetate-oxidizing, sulfur-reducing bacterium Desulfuromonas acetoxidans with green sulfur bacteria, H₂ was produced from acetate via a light-driven sulfur cycle. The sulfur-reducing bacterium could not be replaced by sulfate-reducing bacteria in these experiments. In a coculture of the marine Chlorobium vibrioforme strain 1930 and the sulfur-reducing bacterium Desulfuromonas acetoxidans strain 5071, optimum long-term H₂ production from acetate was obtained with molecular nitrogen as N source, at low light intensity (110 mol · m^-2 · s^-1), in sulfide-reduced mineral medium (2 mM Na₂S) at pH 6.8. Traces of sulfide (10 M) were sufficient to keep the sulfur cycle running. The coculture formed no poly--hydroxyalkanoates (PHA), but 20%–40% polysaccharide per cell dry mass. Per mol acetate added, the coculture formed 3.1 mol of H₂ (78% of the theoretical maximum). Only 8% of the reducing equivalents was incorporated into biomass. The maximum rate of H₂ production was 1300 ml H₂ per day and g cell dry mass.Non-standard abbrevations MOPS 2-(N-morpholino) propane sulfonic acid - MSX Methionine sulfoximine - PHA poly--hydroxyalkanoates     

<Emphasis Type="Italic">Desulfurispirillum alkaliphilum</Emphasis> gen. nov. sp. nov., a novel obligately anaerobic sulfur- and dissimilatory nitrate-reducing bacterium from a full-scale sulfide-removing bioreactor

Strain SR 1^T was isolated under anaerobic conditions using elemental sulfur as electron acceptor and acetate as carbon and energy source from the Thiopaq bioreactor in Eerbeek (The Netherlands), which is removing H₂S from biogas by oxidation to elemental sulfur under oxygen-limiting and moderately haloalkaline conditions. The bacterium is obligately anaerobic, using elemental sulfur, nitrate and fumarate as electron acceptors. Elemental sulfur is reduced to sulfide through intermediate polysulfide, while nitrate is dissimilatory reduced to ammonium. Furthermore, in the presence of nitrate, strain SR 1^T was able to oxidize limited amounts of sulfide to elemental sulfur during anaerobic growth with acetate. The new isolate is mesophilic and belongs to moderate haloalkaliphiles, with a pH range for growth (on acetate and nitrate) from 7.5 to 10.25 (optimum 9.0), and a salt range from 0.1 to 2.5 M Na⁺ (optimum 0.4 M). According to phylogenetic analysis, SR 1^T is a member of a deep bacterial lineage, distantly related to Chrysiogenes arsenatis (Macy et al. 1996). On the basis of the phenotypic and genetic data, the novel isolate is placed into a new genus and species, Desulfurispirillum alkaliphilum (type strain SR^T = DSM 18275 = UNIQEM U250). Nucleotide sequence accession number: the GenBank/EMBL accession number of the 16S rRNA gene sequence of strain SR 1^T is DQ666683.     

Oxidation of H2, organic compounds and inorganic sulfur compounds coupled to reduction of O2 or nitrate by sulfate-reducing bacteria

All of fourteen sulfate-reducing bacteria tested were able to carry out aerobic respiration with at least one of the following electron donors: H₂, lactate, pyruvate, formate, acetate, butyrate, ethanol, sulfide, thiosulfate, sulfite. Generally, we did not obtain growth with O₂ as electron acceptor. The bacteria were microaerophilic, since the respiration rates increased with decreasing O₂ concentrations or ceased after repeated O₂ additions. The amounts of O₂ consumed indicated that the organic substrates were oxidized incompletely to acetate; only Desulfobacter postgatei oxidized acetate with O₂ completely to CO₂. Many of the strains oxidized sulfite (completely to sulfate) or sulfide (incompletely, except Desulfobulbus propionicus); thiosulfate was oxidized only by strains of Desulfovibrio desulfuricans; trithionate and tetrathionate were not oxidized by any of the strains. With Desulfovibrio desulfuricans CSN and Desulfobulbus propionicus the oxidation of inorganic sulfur compounds was characterized in detail. D. desulfuricans formed sulfate during oxidation of sulfite, thiosulfate or elemental sulfur prepared from polysulfide. D. propionicus oxidized sulfite and sulfide to sulfate, and elemental sulfur mainly to thiosulfate. A novel pathway that couples the sulfur and nitrogen cycles was detected: D. desulfuricans and (only with nitrite) D. propionicus were able to completely oxidize sulfide coupled to the reduction of nitrate or nitrite to ammonia. Cell-free extracts of both strains did not oxidize sulfide or thiosulfate, but formed ATP during oxidation of sulfite (37 nmol per 100 nmol sulfite). This, and the effects of AMP, pyrophosphate and molybdate on sulfite oxidation, suggested that sulfate is formed via the (reversed) sulfate activation pathway (involving APS reductase and ATP sulfurylase). Thiosulfate oxidation with O₂ probably required a reductive first step, since it was obtained only with energized intact cells.Abbreviations CCCP carbonyl cyanide m-chlorophenylhydrazone - APS adenosine phosphosulfate or adenylyl sulfate     

Growth yields of green sulfur bacteria in mixed cultures with sulfur and sulfate reducing bacteria

1. Dry weight yields from mixed cultures ofProsthecochloris aestuarii orChlorobium limicola with the sulfur reducingDesulfuromonas acetoxidans were determined on different growth limiting amounts of acetate, ethanol or propanol. The obtained yields agreed well with values predicted from stoichiometric calculations. 2. From mixed cultures of twoChlorobium limicola strains withDesulfovibrio desulfuricans orD. gigas on ethanol as the growth limiting substrate, dry weight yields were obtained as calculated for the complete utilization of the ethanol by the mixed cultures. 3. Dry weight yield determinations for two pure cultures ofChlorobium limicola with different growth limiting amounts of sulfide in the absence and presence of excess acetate confirmed that acetate is incorporated byChlorobium in a fixed proportion to sulfide; compared to the yield in the absence of acetate the yield is increased two to threefold in the presence of acetate. 4. The lowest possible sulfide concentrations necessary for optimal growth of mixed cultures of eitherProsthecochloris orChlorobium withDesulfuromonas on acetate were 7–8 mg H₂S per liter of medium. 5. Doubling times at the growth rate limiting light intensities of 5, 10, 20, 50, 100 and 200 lux were determined under optimal growth conditions for the following phototrophic bacteria:Prosthecochloris aestuarii, Chlorobium phaeovibriodes, Chromatium vinosum andRhodopseudomonas capsulata. Reasonably good growth was still obtained withProsthecochloris at 10 and 5 lux light intensity at which no growth of the purple bacteria could be observed.     

Thiobacillus prosperus sp. nov., represents a new group of halotolerant metal-mobilizing bacteria isolated from a marine geothermal field

From the shallow geothermally heated seafloor at the beach of Porto di Levante (Vulcano, Italy) 8 strains of long, tiny rods were isolated, which represent the first marine metal-mobilizing bacteria. Cells are Gram negative. They grow in a temperature range between 23 and 41°C with an optimum around 37°C at a salt concentration of up to 6.0% NaCl. The isolates are obligately chemolithotrophic, acidophilic aerobes which use sulfidic ores, elemental sulfur or ferrous iron as energy sources and procedure sulfuric acid. They show an upper pH-limit of growth at around 4.5. The G+C content of their DNA is around 64 mol%. Based on the results of the DNA-DNA hybridization they represent a new group within the genus Thiobacillus. Isolate LM3 is described as the type strain of the new species Thiobacillus prosperus.     

New types of acetate-oxidizing,sulfate-reducing Desulfobacter species,D. hydrogenophilus sp. nov., D. latus sp. nov., and D. curvatus sp. nov.

Sulfate-reducing bacteria with oval to rod-shaped cells (strains AcRS1, AcRS2) and vibrio-shaped cells (strains AcRM3, AcRM4, AcRM5) differing by size were isolated from anaerobic marine sediment with acetate as the only electron donor. A vibrio-shaped type (strain AcKo) was also isolated from freshwater sediment. Two strains (AcRS1, AcRM3) used ethanol and pyruvate in addition to acetate, and one strain (AcRS1) grew autotrophically with H₂, sulfate and CO₂. Higher fatty acids or lactate were never utilized. All isolates were able to grow in ammonia-free medium in the presence of N₂. Nitrogenase activity under such conditions was demonstrated by the acetylene reduction test. The facultatively lithoautotrophic strain (AcRS1), a strain (AcRS2) with unusually large cells (2×5 m), and a vibrio-shaped strain (AcRM3) are described as new Desulfobacter species, D. hydrogenophilus, D. latus, and D. curvatus, respectively.     

Fermentation of methoxyacetate to glycolate and acetate by newly isolated strains of Acetobacterium sp.

Three strains of new mesophilic homoacetogenic bacteria were enriched and isolated from sewage sludge and from marine sediment samples with methoxyacetate as sole organic substrate in a carbonate-buffered medium under anoxic conditions. Two freshwater isolates were motile, Gram-positive, non-sporeforming rods. The marine strain was an immotile, Gram-positive rod with a slime capsula. All strains utilized only the methyl residue of methoxyacetate and released glycolic acid. They also fermented methyl groups of methoxylated aromatic compounds and of betaine to acetate with growth yields of 6–10 g dry matter per mol methyl group. H₂/CO₂, formate, methanol, hexamethylene tetramine, as well as fructose, numerous organic acids, glycerol, ethylene glycol, and glycol ethers were fermented to acetate as well. High activities of carbon monoxide dehydrogenase (0.4–2.2 U x mg protein^–1) were detected in all three isolates. The guanine-plus-cytosine-content of the DNA of the freshwater isolates was 42.7 and 44.4 mol %, with the marine isolate it was 47.7 mol %. The freshwater strains were assigned to the genus Acetobacterium as new strains of the species A. carbinolicum. One freshwater isolate, strain KoMac1, was deposited with the Deutsche Sammlung von Mikroorganismen GmbH, Braunschweig, under the number DSM 5193.     

Fermentation of trihydroxybenzenes by Pelobacter acidigallici gen. nov. sp. nov., a new strictly anaerobic,non-sporeforming bacterium

Five strains of rod-shaped, Gram-negative, non-sporing, strictly anaerobic bacteria were isolated from limnic and marine mud samples with gallic acid or phloroglucinol as sole substrate. All strains grew in defined mineral media without any growth factors; marine isolates required salt concentrations higher than 1% for growth, two freshwater strains only thrived in freshwater medium. Gallic acid, pyrogallol, 2,4,6-trihydroxybenzoic acid, and phloroglucinol were the only substrates utilized and were fermented stoichiometrically to 3 mol acetate (and 1 mol CO₂) per mol with a growth yield of 10g cell dry weight per mol of substrate. Neither sulfate, sulfur, nor nitrate were reduced. The DNA base ratio was 51.8% guanine plus cytosine. A marine isolate, Ma Gal 2, is described as type strain of a new genus and species, Pelobacter acidigallici gen. nov. sp. nov., in the family Bacteroidaceae. In coculture with Acetobacterium woodii, the new isolates converted also syringic acid completely to acetate. Cocultures with Methanosarcina barkeri converted the respective substrates completely to methane and carbon dioxide.     

Reduced inorganic sulfur oxidation supports autotrophic and mixotrophic growth of Magnetospirillum strain J10 and Magnetospirillum gryphiswaldense

Magnetotactic bacteria are present at the oxic–anoxic transition zone where opposing gradients of oxygen and reduced sulfur and iron exist. Growth of non‐magnetotactic lithoautotrophic Magnetospirillum strain J10 and its close relative magnetotactic Magnetospirillum gryphiswaldense was characterized in microaerobic continuous culture. Both strains were able to grow in mixotrophic (acetate + sulfide) and autotrophic (sulfide or thiosulfate) conditions. Autotrophically growing cells completely converted sulfide or thiosulfate to sulfate and produced 7.5 g dry weight per mol substrate at a maximum observed growth rate of 0.09 h^?1 for strain J10 and 0.07 h^?1 for M. gryphiswaldense. The respiratory activity for acetate was repressed in autotrophic and also in mixotrophic cultures, suggesting acetate was used as C‐source in the latter. We have estimated the proportions of substrate used for assimilatory processes and evaluated the biomass yields per mol dissimilated substrate. The yield for lithoheterotrophic growth using acetate as the C‐source was approximately twice the autotrophic growth yield and very similar to the heterotrophic yield, showing the importance of reduced sulfur compounds for growth. In the draft genome sequence of M. gryphiswaldense homologues of genes encoding a partial sulfur‐oxidizing (Sox) enzyme system and reverse dissimilatory sulfite reductase (Dsr) were identified, which may be involved in the oxidation of sulfide and thiosulfate. Magnetospirillum gryphiswaldense is the first freshwater magnetotactic species for which autotrophic growth is shown.     

Isolation and Characterization of Strains CVO and FWKO B, Two Novel Nitrate-Reducing, Sulfide-Oxidizing Bacteria Isolated from Oil Field Brine

Bacterial strains CVO and FWKO B were isolated from produced brine at the Coleville oil field in Saskatchewan, Canada. Both strains are obligate chemolithotrophs, with hydrogen, formate, and sulfide serving as the only known energy sources for FWKO B, whereas sulfide and elemental sulfur are the only known electron donors for CVO. Neither strain uses thiosulfate as an energy source. Both strains are microaerophiles (1% O₂). In addition, CVO grows by denitrification of nitrate or nitrite whereas FWKO B reduces nitrate only to nitrite. Elemental sulfur is the sole product of sulfide oxidation by FWKO B, while CVO produces either elemental sulfur or sulfate, depending on the initial concentration of sulfide. Both strains are capable of growth under strictly autotrophic conditions, but CVO uses acetate as well as CO₂ as its sole carbon source. Neither strain reduces sulfate; however, FWKO B reduces sulfur and displays chemolithoautotrophic growth in the presence of elemental sulfur, hydrogen, and CO₂. Both strains grow at temperatures between 5 and 40°C. CVO is capable of growth at NaCl concentrations as high as 7%. The present 16s rRNA analysis suggests that both strains are members of the epsilon subdivision of the division Proteobacteria, with CVO most closely related to Thiomicrospira denitrifcans and FWKO B most closely related to members of the genus Arcobacter. The isolation of these two novel chemolithotrophic sulfur bacteria from oil field brine suggests the presence of a subterranean sulfur cycle driven entirely by hydrogen, carbon dioxide, and nitrate.     

Oxidation of organic compounds to CO2 with sulfur or thiosulfate as electron acceptor in the anaerobic hyperthermophilic archaea Thermoproteus tenax and Pyrobaculum islandicum proceeds via the citric acid cycle

The oxidation of organic compounds with elemental sulfur or thiosulfate as electron acceptor was studied in the anaerobic hyperthermophilic archaea Thermoproteus tenax and Pyrobaculum islandicum. T. tenax was grown on either glucose or casamino acids and sulfur; P. islandicum on peptone and either elemental sulfur or thiosulfate as electron acceptor. During exponential growth only CO₂ and H₂S rather than acetate, alanine, lactate, and succinate were detected as fermentation products of both organisms; the ratio of CO₂/H₂S formed was 1:2 with elemental sulfur and 1:1 with thiosulfate as electron acceptor. Cell extracts of T. tenax and P. islandicum contained all enzymes of the citric acid cycle in catabolic activities: citrate synthase, aconitase, isocitrate dehydrogenase (NADP⁺-reducing), oxoglutarate: benzylviologen oxidoreductase, succinyl-CoA synthetase, succinate dehydrogenase, fumarase and malate dehydrogenase (NAD⁺-reducing). Carbon monoxide dehydrogenase activity was not detected. We conclude that in T. tenax and P. islandicum organic compounds are completely oxidized to CO₂ with sulfur or thiosulfate as electron acceptor and that acetyl-CoA oxidation to CO₂ proceeds via the citric acid cycle.